Polymer matrix composites with nano-scale reinforcements

ABSTRACT

Embodiments of the present invention provide polymer matrix nanocomposites reinforced with nano-scale materials such as nanoparticles and carbon nanotubes and methods of fabricating. The nanomaterials are provided within relatively low weight fractions, for example in the range of approximately 0.01 to about 0.4% by weight and distributed within the matrix by a magnetic mixing procedure to provide substantially uniform reinforcement of the nanocomposites. Advantageously, these nanocomposites provide significantly enhanced tensile strength, strain to failure, and fracture toughness over corresponding neat matrices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Application No. 60/753,006 filed on Dec. 21, 2005,entitled TOUGHENING OF EPOXY USING SIC NANOPARTICLES, U.S. ProvisionalApplication No. 60/753,155 filed on Dec. 20, 2005, entitled TOUGHENINGOF POLYESTER RESIN USING TIO2 NANOPARTICLES, and U.S. ProvisionalApplication No. 60/753,496 filed on Dec. 22, 2005 entitled SUPERPERFORMING CNT/EPOXY RESIN, the entirety of each of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED R&D

This invention was made with Government support under Contract NumberN00014-05-1-05086 awarded by ONR (Office of Naval Research). TheGovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Certain embodiments of the present invention relate generally tocomposite materials and, in particular, relate to systems and methodsfor reinforcement of polymer matrices with low concentrations ofuniformly dispersed nanomaterials.

2. Description of the Related Art

Composite materials have been developed to meet increasing demands formaterials possessing a broad array of desirable properties. Compositesare material systems which combine two or more distinct materials, eachwith its own distinctive, desirable properties, to create a new materialwith properties that may not be present, or to the same extent, in thecomponents alone. Composite materials, broadly, possess at least twophases—a reinforcement and a matrix. The reinforcement is a materialwhich is embedded within the matrix. In general, the reinforcingmaterial and the matrix material may comprise any combination of metals,ceramics, or polymers.

In one example polymer matrix composites (PMCs) combine strongreinforcing fibers within a polymer matrix. Advantageously, PMCs possessrelatively high strength, low weight, and corrosion resistance, whichhas spurred their development for applications in aerospace, sportinggoods, automotive, and other industries where environmental and weightconcerns play a key role in design considerations. Furthermore, thesematerials may be fabricated at relatively low costs, further increasingtheir desirability.

The use of polymers as matrix materials in composites demands excellentmechanical performance over a large range of temperatures. A limitingfactor in the use of polymer matrices, however, is their relativebrittleness and tendency to exhibit microcracking at low levels ofstrain. These microcracks may coalesce under load, forming macrocracks,and due to the relative brittleness of the polymer matrices, result incatastrophic failure with little warning. And while design strains canbe kept to low levels to prevent microcracking and catastrophic failure,additional composite material is required to bear the applied load,increasing the total weight of the composite structures.

To mitigate the brittleness and microcracking of polymer matrices,researchers have experimented with the addition of nano-scale materialsto polymer matrices in order to improve their strain to failure andfracture toughness. Nano-scale materials possess great promise ascomposite reinforcements. In general, the greater the surface to volumeratio of a reinforcement, the greater the effectiveness of a material asa reinforcement, and nano-scale materials have a high surface to volumeratio owing to their small size.

Agglomeration, however, has been a significant obstacle to the use ofnano-scale materials as reinforcements. Small particles, those having adiameter less than approximately 1 μm, have a strong tendency toagglomerate, or group together, under the influence of Van der Waalsforces. In the case of submicron particles, these forces are strongerthan gravitational forces and gives rise to spontaneous agglomeration.Agglomerated particles contain small voids which are difficult for thereinforcing matrix to enter by capillary action during processing of thecomposite. Thus, a reinforced composite formed with agglomeratespossesses voids which can act as flaws that, instead of benefiting thecomposite, are detrimental to its mechanical properties. In particular,a number of studies have observed that the addition of nano-scalematerials may improve some mechanical properties, such as fracturetoughness or stiffness, while other mechanical properties, such asstrength, are detrimentally affected to the point where properties ofthe composite are less than that of the matrix alone.

Thus, there is need for an improved systems and methods of manufacturingcomposites reinforced with nano-scale materials and, in particular,polymer matrix composites which have improved fracture propertieswithout incurring substantially detrimental impact upon other mechanicalproperties.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a composite material,comprising a matrix material and a reinforcement material. Thereinforcement material preferably has at least one dimension ofapproximately 100 nm or less and is dispersed substantially uniformlywithin the matrix material. In one embodiment, the reinforcementmaterial is present in a concentration between approximately 0.01% toless than about 1% on the basis of the weight of the composite material.In one embodiment, the matrix is reinforced with nanoparticles in aconcentration of between about 0.1% and 0.5% by weight. In a furtherembodiment, the nanoparticles are provided in the range of approximately0.2 to less than 0.5 wt %. In another embodiment, the nanoparticles areprovided in the range of approximately 0.2 to 0.4 wt %. In an additionalembodiment, the nanoparticles are provided in the range of approximately0.2 to less than 0.4 wt %. In another embodiment, the matrix isreinforced with nanotubes in a concentration of between about 0.01% and0.1% by weight.

Another embodiment of the present invention provides a SiC-reinforcedcomposite material. The SiC-reinforced composite comprises a matrixmaterial such as epoxy and SiC nanoparticles. The SiC nanoparticles havea diameter of about 100 nm or less and are present in a concentration ofbetween approximately 0.1 to less than about 1%, more preferably lessthan about 0.9%, and even more preferably between about 0.2 to 0.5% orabout 0.2 to 0.4%, on the basis of the weight of the composite material.

In another embodiment, a composite material is provided with titaniumdioxide nanoparticle reinforcement. For example, the composite maycomprise a polyester resin and about 0.1 wt % to 0.4 wt %, morepreferably about 0.2 wt % titanium dioxide nanoparticles.

In another embodiment, a composite material is provided with carbonnanotube reinforcement. For example, the composite may comprise an epoxyresin and a very low weight percentage of carbon nanotubes, e.g., about0.01% to 0.1% by weight. The nanotubes may have a width or diameter ofabout 100 nm or less.

In other embodiments, any suitable nano-material loading that causes anincrease in ultimate strength of about 10% or more, more preferablyabout 25% or more, and/or an increase in strain to failure of about 50%or more, may be utilized. In other embodiments, any suitablenano-material loading that causes about a two fold or more rise inG_(IC), and/or about a 20% or more rise in K_(IC), may be utilized.

In another embodiment of the present invention, a method ofmanufacturing a nanocomposite, such as a polymer nanocomposite, isprovided. The method comprises providing a reinforcement material havingat least one dimension of approximately 100 nm or less and in aconcentration between approximately 0.01 wt % to less than about 1 wt %on the basis of the weight of the composite material. The method furthercomprises providing a matrix material. The method additionally comprisesdispersing the reinforcement material within the matrix material using amagnetic mixing system. A curing agent may be added to thereinforcement-matrix mixture, and the dispersed reinforcement-matrixmixture may be placed within a mold. The method further comprises curingthe matrix material.

In a further embodiment, the reinforcement material comprises carbonnanotubes (CNTs), which are first added to a substantially volatilesolvent such as ethyl alcohol (EtOH). A sonication technique is thenutilized to disperse the CNT's within the solvent. The dispersedsolution is then mixed with the resin using the magnetic mixingtechnique described above. A further sonication technique may beutilized to disperse the CNTs within the resin.

In another embodiment, a nanomaterial-reinforced nail polish isprovided. The nail polish may be a composite comprising one or more nailpolish coatings and a nanomaterial reinforcement material in one or moreof the coatings. In one embodiment, the nanomaterial reinforcement isprovided in a concentration of about 0.2 to 0.4 wt % on the basis of theweight of the composite.

These and other objects and advantages of preferred embodiments of thepresent invention will become apparent from the following descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B are schematic illustrations of embodiments of nanocompositeshaving substantially uniformly distributed nanomaterials: (1A)Nanoparticles; (1B) Carbon Nanotubes;

FIGS. 2A and 2B are embodiments of methods for manufacture of thenanocomposites having substantially uniformly distributed nanoparticles(FIG. 2A) and carbon nanotubes (FIG. 2B);

FIGS. 2C-E illustrate embodiments of nanocomposites containing a SiCnanoparticle reinforcement at loadings of 0.2% (2C), 1% (2D), and 1.5%(2E);

FIGS. 3A-3B are schematic illustrations of test specimens formeasurement of tensile and fracture properties of the nanocomposites ofFIGS. 1A-B;

FIGS. 4A-B are micrographs of as-processed neat matrix and SiCreinforced nanocomposites;

FIGS. 5A-E are micrographs of embodiments of an as-processed neat matrixand SiC nanocomposites at loadings of approximately 0.2, 0.5, 1, and1.5%, respectively;

FIGS. 6A-D present measured the tensile properties for embodiments ofSiC nanocomposites and the corresponding neat matrix: (6A) Tensilestress-strain response; (6B) Young's Modulus as a function of SiCloading; (6C) Ultimate tensile strength as a function of SiC loading;(6D) Strain to failure as a function of SiC wt %;

FIGS. 7A-B present the measured fracture properties for SiCnanocomposites and the corresponding neat matrix: (7A) G_(IC) as afunction of SiC loading; (6B) K_(IC) as a function of SiC loading;

FIGS. 8A-C are micrographs of SENB fracture surfaces of embodiments of aneat matrix (FIG. 8A) and SiC reinforced nanocomposites having loadingfractions of 0.2% (FIG. 8B) and 1% (FIG. 8C);

FIGS. 9A-B are micrographs of embodiments of an as-processed neat matrixand a 0.2% TiO₂-reinforced nanocomposite;

FIGS. 10A-C present measured tensile properties for embodiments of aTiO₂ nanocomposites and the corresponding neat matrix: (10A)Stress-strain response; (10B) Strain to failure as a function of TiO₂loading; (10C) Ultimate tensile strength as a function of TiO₂ wt %;

FIGS. 11A-B present measured fracture properties for embodiments of aTiO₂ nanocomposites and the corresponding neat matrix; (11A) G_(IC) as afunction of TiO₂ loading; (11B) K_(IC) as a function of TiO₂ loading;

FIGS. 12A-B are photographs of embodiments of an as-processed neatmatrix and an approximately 0.02 wt % nanotube-reinforced nanocomposite;

FIGS. 13A-C present measured tensile properties for embodiments of CNTnanocomposites and the corresponding neat matrix: (13A) Stress-strainresponse; (13B) Strain to failure as a function of CNT loading; (13C)Ultimate tensile strength as a function of CNT wt %;

FIGS. 14A-B present measured fracture properties for embodiments of CNTnanocomposites and the corresponding neat matrix: (14A) K_(IC) as afunction of CNT loading; (14B) G_(IC) as a function of CNT loading and;

FIGS. 15A-B present micrographs of artificial nails painted with asingle coating of nail polish; (15A) Unreinforced; (15B) Reinforced withTiO₂ nanoparticles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like numerals referto like parts throughout. Throughout the disclosure, reference toloadings of nano-scale reinforcing materials are provided as wt % on thebasis of the total weight of the nanocomposite.

It has been discovered that low weight percentage loadings of nano-scalereinforcing materials, such as nanoparticles and carbon nanotubes, topolymer matrices may be mechanically dispersed during fabrication ofpolymer nanocomposites to provide nanocomposites which are substantiallyfree of agglomerates. As described herein, nano-scale reinforcingmaterials according to certain embodiments have at least one dimensionof about 100 nm or less. More preferably, the nano-scale reinforcingmaterials have two dimensions of about 100 nm or less. It will beappreciated, however, that other embodiments of the invention may not belimited to these particular dimensions.

In one non-limiting embodiment, the nano-scale reinforcement maycomprise nanoparticles having a diameter of approximately 100 nm orless. Such nanoparticles may be elemental, oxide or non-oxide ceramics,metallic alloys and intermetallics, or combinations thereof. Morepreferably, certain embodiments described herein are directed to ceramicor elemental nanoparticle reinforcements. In a further non-limitingembodiment, the nano-scale reinforcement may comprise carbon nanotubeshaving a diameter of approximately less than 100 nm. Advantageously,polymer nanocomposites so fabricated possess significantly enhancedfracture toughness while concurrently preserving or improving theirtensile properties, such as tensile strength, elastic modulus, andstrain to failure, over comparable un-reinforced polymers.

FIGS. 1A-B present embodiments of a nanocomposite 100 havingsubstantially uniformly distributed nano-scale reinforcements 102possessing substantially low levels of agglomerates, designed forimproved fracture toughness and strain to failure. In certainembodiments, the nanocomposite 100 comprises a polymer matrix 104reinforced with nanoparticles 102A (FIG. 1A) or carbon nanotubes 102B(FIG. 1B), which are substantially uniformly distributed within thematrix 104 during processing. In one embodiment, the weight fraction ofthe nano-scale reinforcement 102, or loading, is relatively low, on theorder of approximately 0.02% to 1.5%. In a preferred embodiment, theloading is in the range of approximately 0.02% to less thanapproximately 1 wt %. Higher loadings may be used, as necessary,depending on factors such as the nano-scale reinforcement 102 and theability to disperse the reinforcement within the matrix 104. In furtherembodiments, nano-reinforcement 102 is substantially dispersed using amagnetic mixing process, as discussed below. So fabricated, i.e., usinglow loading percentages and mixing as disclosed above here and withproper mixing technique as employed here, embodiments of thenanocomposites 100 possess less substantially no agglomerated nano-scalereinforcements 102 (as shown in FIGS. 2C-E below), where theagglomerates comprise a plurality of individual nano-scalereinforcements 102 in contact with one another, on the basis of thetotal amount of reinforcing material 102 added. Advantageously,nanocomposites 100 within these loading ranges may concurrentlydemonstrate significantly improved tensile strength, strain to failure,and fracture toughness over comparable, un-reinforced neat matrices, asdiscussed in detail below.

In a non-limiting embodiment, the matrix 104 may comprise a polymer. Ina preferred embodiment, the polymer may comprise a high temperatureepoxy based on diglycidyl ether of bisphenol A (DGEBPA) andalkylglycidyl ether (AE), or DGEBPA-AE. In an alternative embodiment,the polymer matrix may comprise a polyester. In further alternativeembodiments, the matrix may comprise other polymers or polymerprecursors such as pre-ceramic polymers. One preferred embodiment of apre-ceramic polymer comprises KiON CERASET®.

In non-limiting embodiments, the nano-scale reinforcements 102 maycomprise nanoparticles 102A of elements, oxide and non-oxide ceramics,metallic alloys and intermetallics, or combinations thereof. Inpreferred embodiments, the nanoparticles 102A may comprise TitaniumDioxide (TiO₂), Yttrium Oxide (Y₂O₃), Zinc Oxide (ZnO), Silicon Carbide(SiC), and Carbon (C) or combinations thereof. In one particularlypreferred embodiment, a single nanoparticle species is utilized as areinforcement. The nanoparticles 102A may further range in size fromapproximately 15 nm to 45 nm and are provided in a relatively low weightfraction with respect to the total weight of the nanocomposite. In oneembodiment, the nanoparticles 102A are provided in the range ofapproximately 0.2 to 1.5 wt %, more preferably less than about 1 wt %.In an alternative embodiment, the nanoparticles 102A are provided in therange of approximately 0.2 to 0.5 wt %. In a further embodiment, thenanoparticles 102A are provided in the range of approximately 0.2 toless than 0.5 wt %. In another embodiment, the nanoparticles 102A areprovided in the range of approximately 0.2 to 0.4 wt %. In an additionalembodiment, the nanoparticles 102A are provided in the range ofapproximately 0.2 to less than 0.4 wt %.

In a further non-limiting embodiment, the nano-scale reinforcement 102may comprise carbon nanotubes 102B. Carbon nanotubes 102B are arelatively new configuration of carbon which exhibit a variety ofsuperior mechanical properties. Progress made in the production andpurification of nanotubes in recent years has made it practical toconsider nanotubes 102B as reinforcements in composite materials. Withrespect to their mechanical properties, nanotubes 102B exhibitextraordinary strength, on the order of tens of GPa, and elastic modulion the order of about 1 TPa. Additionally, nanotubes 102B may undergoplastic deformation at elevated tensile stresses, allowing for somedegree of permanent deformation. This deformation begins at strains ofapproximately 5% and can increase the maximum strain the tube undergoesbefore fracture. These mechanical properties make CNTs 102B excellentcandidates as reinforcing materials. Further, carbon nanotubes 102Bpossess a relatively low density for a solid, approximately 1.3-1.4g/cm³. This provides CNTs 102B a specific strength which is very high,also making them very attractive for use in composite materials, whichare often utilized in applications where weight is a key designconsideration.

In one embodiment, the CNTs 102B may comprise single walled carbonnanotubes (SWNTs) and/or multiple walled carbon nanotubes, MWNTs. SWNTscan be conceptualized as wrapping a one-atom-thick layer of graphitecalled graphene into a seamless cylinder, while MWNTs are multiplelayers of graphite rolled upon themselves to form a tube shape. CNTs102B possess diameters on the order of about 1 nm with tube lengths thatcan be many thousands of times larger. In one preferred embodiment, theCNTs 102B comprise SWNTs having lengths in the range of approximately 1to 500 micrometers. In further embodiments, the SWNTs are provided atloadings of approximately 0.02 to 0.1 wt %.

As discussed in greater detail with respect to the examples below, theuse of substantially uniformly dispersed nanoparticles or CNTs 102B asreinforcements for nanocomposites 100 provides improved fracturetoughness while concurrently preserving or improving the tensileproperties of the nanocomposite 100, such as tensile strength and strainto failure, compared to un-reinforced neat matrices. In one embodiment,the carbon nanotubes 102B are provided in the range of approximately0.01 to 0.1 wt %. In further alternative embodiments, the loadingfraction of the nano-scale materials may be adjusted within this rangeas generally understood by one of skill in the art so as to providenanocomposites 100 having combinations of mechanical properties whichare elevated above those of the un-reinforced matrix.

One embodiment of a method 200 by which nanocomposites havingsubstantially uniformly distributed nanoparticles may be fabricated isillustrated in FIG. 2A. In a first step 202, a predetermined, low weightloading of nanoparticles is added to a matrix material which iscontained within a beaker or other vessel. The nanoparticle material,size, and loading fraction may be varied, as discussed above. In anon-limiting example of the method, the matrix material will be referredto as a polymer resin, however, as discussed above, the matrix is notlimited to polymer materials.

In a second step 204, the nanoparticle-resin mixture is mechanicallymixed using a magnetic mixing system. In one embodiment, the magneticmixing system comprises a magnetic stirring device and a magneticstirring bar. The magnetic stirring bar is added to the beaker and thebeaker is placed upon the magnetic stirring device. Activation of thestirring device causes the stirring bar to rotate using non-contactmagnetic torque, providing mechanical mixing of the nanoparticle-matrixmixture. The magnetic mixer is operated according to a predeterminedschedule of mixing time and rpm, based on the matrix and nanoparticlematerials. In one embodiment, approximate values for magnetic mixingtimes and speeds which may provide a substantially uniform distributionof nanoparticles within the matrix are provided as illustrated below inTable I and discussed in greater detail below with respect to theexamples.

TABLE I Nanoparticle mixing schedule Magnetic Manual Mixing Mixing CurePost-Cure Time Time Time Temp Time Nanoparticle Matrix RPM (h) (min) (h)(° C.) (h) SiC DGEBPA- 1200 10 20-30 3 120 1 AE TiO₂ Polyester 1500 7005 4 N/A N/AIn alternative embodiments, the magnetic mixing speed and time may beincreased or decreased as necessary

In a third step 206, a curing agent such as a hardener is added to theresin-nanoparticle mixture. In one embodiment, the resin to hardenerratio is provided in accordance with the manufacturer's instruction. Ina non-limiting example, a resin to hardener ratio of approximately 3:1may be used with DGEBPA-AE epoxy, while a ratio of approximately 30 mLto 10 drops may be used with polyester resins. In alternativeembodiments, the ratio of hardener to resin may be increased ordecreased as necessary to cure the nanocomposite.

In a fourth step 210 of the method 200, the nanoparticle-resin mixtureis subjected to a manual mixing procedure for a predetermined time. Themixture is stirred with a stirring rod or other device or implementgenerally known in the art for hand mixing. The mixing is performed at alinear speed of approximately 1 cm/sec to avoid foaming of the mixture.In one embodiment, the manual mixing is conducted over the time range ofapproximately 5 to 30 minutes, as illustrated in Table I above. Inalternative embodiments, the manual mixing time and linear speed may beincreased or decreased.

In a fifth step 212, following mixing, the nanoparticle-resin mixture ispoured into a polished aluminum mold or other mold and allowed to curein air. Cure times and temperatures are, in one embodiment, provided inaccordance with the resin manufacturer's instructions. In oneembodiment, the nanocomposite is cured at approximately room temperaturefor times ranging between approximately 3 hours to 4 hours. Inalternative embodiments, the cure time and temperature may be increasedor decreased.

In alternative embodiments, the cured nanocomposite may be subjected toa sixth step 214 comprising a post-cure at elevated temperature. In oneembodiment, the post-cure schedule comprises heat treatment atapproximately 120° C. for approximately one hour.

In one embodiment, a method 216 of manufacture of CNT reinforcednanocomposites (FIG. 2A) builds upon the framework of the first method200 described above (FIG. 2A), using additional steps. The second method216 utilizes these additional steps in order to disperse the CNTs, as ahigh degree of adhesion is present between CNTs which can make SWNTsdifficult to separate into single units from aggregates and clusterswithin the viscous polymer matrix during processing.

In a first step 220 of the second method 216, a predetermined, lowweight loading of nanotubes is added to a substantially volatile solventwhich is contained within a beaker or other vessel. In one embodiment,ethyl alcohol (EtOH) is a used as the volatile solvent, owing to itsrelatively high volatility and its low viscosity compared to the polymerresin. However, in alternative embodiments, other volatile solvents maybe used, such as, in non-limiting embodiments, other alcohols andacetone. The size, and loading fraction of the nanotubes, may be varied,as discussed above.

In a second step of the method 222, the beaker is subsequently placed ina sonication water bath which utilizes sound waves to cause vibrationswithin the water which are transferred to the CNT-EtOH solution. In oneembodiment, sonication is performed for approximately five hours at roomtemperature to generate a substantially uniform distribution of CNTswithin the EtOH.

Subsequently, in a third step 224 of the method 216, the dispersedEtOH-CNT solution is mixed with the resin using the magnetic mixingtechnique discussed above. In one embodiment, the resin comprisesDGEBPA-AE. In a further embodiment, the magnetic stirring bar rotates atapproximately 700 RPM at approximately room temperature forapproximately 15 hours At the end of the mixing period, further stirringis performed at approximately the same speed at about 80° C. forapproximately 45 minutes. In this manner, the volatile EtOH or other CNTdispersion solvent is substantially removed from the resin-CNT mixture.

In a fourth step 226 of the second method 216, sonication is performed asecond time as in order to ensure substantially uniform dispersion ofthe CNTs within the resin prior to curing. In one embodiment, theresin-CNT mixture is sonicated for approximately 25 minutes.

In a fifth step 230 of the second method 216, the hardener is added tothe resin-CNT mixture. In a non-limiting example, a resin to hardenerratio of approximately 3:1 is used with DGEBPA-AE epoxy. In alternativeembodiments, the ratio of hardener to resin may be increased ordecreased.

In a sixth step 232 of the second method 216, the mixture is subjectedto a manual mixing procedure for a predetermined time. The mixture isstirred with a stirring rod or other device generally known in the artfor mixing by hand. The mixing is performed at a linear speed ofapproximately 1 cm/sec to avoid foaming of the mixture. In oneembodiment, the manual mixing is conducted for approximately 20-30minutes. In alternative embodiments, the manual mixing time and linearspeed may be increased or decreased, as necessary to ensuresubstantially uniform distribution of the CNTs within the resin.

In a seventh step 234, following mixing, the mixture is poured into apolished aluminum mold or other mold and allowed to cure in air. Curetimes and temperatures are, in one embodiment, provided in accordancewith the polymer manufacturer's instructions. In one embodiment, thenanocomposite is cured at approximately room temperature forapproximately 5 hours. In alternative embodiments, the cure time andtemperature may be increased or decreased.

In an eighth step 236 of the method 216, the nanocomposite may beoptionally subjected to a post-cure treatment. In one embodiment, thepost-cure comprises heat treatment at approximately 120° C. forapproximately one hour.

Nanocomposites fabricated according to the methods 200, 216 may possesssubstantially low levels of agglomerates. FIG. 2C-E present embodimentsof SiC-reinforced nanocomposites at loading fractions of 0.2, 1, and 1.5wt %, respectively. It is observed that at 0.2 wt %, the nano-scalereinforcements 102 are substantially uniformly distributed andunagglomerated 240. In contrast, at loadings of 1 and 1.5 wt %,relatively large agglomerates 242 of the nano-scale materials areobserved, with the size of the agglomerates 242 increasing withincreasing loading.

EXAMPLES Bulk Nanocomposites

In the following examples, testing is performed on nanocompositesreinforced using nanoparticles and carbon nanotubes and theircorresponding neat matrices in order to illustrate the propertyimprovements which may be achieved in these composites over comparableun-reinforced polymer materials. In particular, the tensile and fractureproperties are examined. The nanocomposites tested are formed asdescribed above, using two commonly available nanoparticles, SiC andTiO₂, and SWNTs with matrices of DGEBPA-AE and polyester.Advantageously, these polymers are representative of those commonly usedin present composites manufacturing, room temperature cure polymers andhigh temperature cure epoxies, and thus provide results which arerelevant to present applications. To assess the influence of thenano-scale reinforcements on the performance of the nanocomposites, neatmatrices were formed without the reinforcements.

Bulk Nanocomposites: Mechanical Testing

In the examples, mechanical characterization focuses on evaluation ofthe tensile and fracture properties of embodiments of the nanocompositeand corresponding neat matrix. Tensile properties of the neat matricesand nanocomposites are measured according to ASTM D638, Standard TestMethod for Tensile Properties of Plastics. Dogboned uniaxial tensilespecimens 300 of each material are provided having dimensions consistentwith the standard (FIG. 3A). Tests are conducted using an Instron™ type4200 universal testing machine and the load (P) and displacement (d) arerecorded. Strain along the loading axis of the composite is alsomeasured and recorded using strain gauges 302 bonded to the specimen300. From the stress-strain data, the elastic modulus, yield stress,ultimate stress, and strain to failure are determined in accordance withthe standard. For each material condition, at least four specimens weretested.

The fracture properties of the nanocomposites and neat matrices arequantified by measurement of the critical stress intensity factor,K_(IC), and the critical strain energy release rate, G_(IC), throughsingle edge notch bend tests (SENB) performed according to ASTM D5045,“Standard Test Method for Plane-Strain Fracture Toughness and StrainEnergy Release Rate of Plastic Materials.” K_(IC) and G_(IC) arematerial properties which provide a measure of the resistance of amaterial to crack propagation in the presence of a sharp crack in a ModeI, or tensile mode. K_(IC) scales with the load at which crackpropagation begins, while G_(IC) scales with the fracture work.

The SENB test specimen 304 is illustrated in FIG. 3B. The SENB specimen304 comprises a generally rectangular beam having a length L, a width,W, a thickness B. A notch 306 of length, a, is introduced into the SENBspecimen 304, approximately centered on the length, extending in thewidth direction. The SENB specimen 304 is loaded in flexure at a rate ofapproximately 0.25 mm/min, approximately opposite the notch 306, andsupported at positions flanking the notch 306, separated by a spanlength S, and centered approximately on the notch 306. So configured,the SENB specimen 304 is loaded in compression, with monotonicallyincreasing load, P, causing the specimen 304 to bend downwards. Load anddisplacement of the specimen 304 at the load line are measured and thenotch 306 is monitored to identify the load at which crack propagationinitiates at the notch 306.

K_(IC) is calculated according to the standard ASTM D5045. The validityof the plain strain K_(IC) value so calculated is verified usingEquation (1):

$\begin{matrix}{B,a,{{{and}\mspace{11mu}( {W - a} )} > {2.5( \frac{K_{IC}}{\sigma_{y}} )^{2}}}} & (1)\end{matrix}$where σ_(y) is the yield stress, calculated from a 0.2% offset appliedto the tensile stress-strain curve.

G_(IC) is calculated using the load-displacement response of the SENBspecimen 306 according to ASTM D5045. A corrected displacement isobtained from an indentation test outlined in the standard. G_(IC) isthen determined from the energy given by the area under theload-displacement curve using Equation (2):

$\begin{matrix}{G_{IC} = \frac{U}{{BW}\;\varphi}} & (2)\end{matrix}$where U is the corrected energy and φ is an energy calibration factor.

In some cases, fracture surfaces of the SENB specimens 306 are alsoexamined using a Hitachi S-4800 Field Emission Scanning ElectronMicroscope (SEM). Fracture surfaces are covered with a thin layer ofsputtered gold and imaged in the SEM using an acceleration voltage ofapproximately 1 kV.

Bulk Nanocomposites: SiC Nanoparticle Reinforced Nanocomposites

Investigations below illustrate the effect of inclusion of low loadingsof SiC nanoparticles on the tensile and fracture properties ofembodiments of the nanocomposites and comparable un-reinforced neatmatrices. An epoxy based on DGEBPA-AE is mixed with SiC nanoparticles(Accumet Materials Co.). The SiC nanoparticles possess a density ofabout 3.22 g/cm³, a surface area of 70-90 m²/g, and a size n rangingbetween about 45 to 55 nm. Four weight fractions of SiC nanoparticlesare investigated: 0.2, 0.5, 1, and 1.5%. The SiC nanoparticles are addedto the epoxy resin and the mixture is stirred using the magnetic mixingdevice for approximately 10 hours at about 1200 rpm in order to dispersethe nanoparticles within the matrix resin. Subsequently, the hardener isadded to the homogeneous mixture in an epoxy to hardener ratio ofapproximately 3:1. Following addition of the hardener, the mixture isstirred manually for approximately 20-30 minutes at a rate ofapproximately 1 cm/sec. Following mixing, the mixture is poured into apolished aluminum mold and allowed to cure in air at room temperaturefor approximately 3 hours. Subsequently, the cured composites arepost-cured for approximately 1 hr at about 120° C. in air. To quantifythe effect of low SiC nanoparticle loading on the performance of thecomposite, an epoxy panel without SiC nanoparticles is also manufacturedusing the same procedure.

FIGS. 4A-B illustrates the appearance of the as-processed neat matrixand SiC-reinforced nanocomposite of approximately 0.2% to 0.4% SiC byweight. It is apparent from the micrographs of FIG. 4A-B, the SiCreinforced nanocomposites (4B) present an ash-colored appearance, whilethe neat matrix (4A) is approximately transparent. In one aspect, thisresult demonstrates that even small loadings of nanoparticles aresufficient to change the optical properties of the nanocomposite fromthose of the neat matrix from transparent to opaque. In another aspect,this result indicates that the mixing procedure described above providessubstantially homogeneous dispersion of the nanoparticles within theresin, as the appearance of the nanocomposite is approximately uniform.

The appearance of an as-processed neat matrix and SiC nanocomposites atloadings of approximately 0.2, 0.5, 1, and 1.5% are provided in FIGS.5A-E. Examining the appearance of the nanocomposites, it is apparentthat poorer dispersion of the SiC nanoparticles takes place at thehigher loading fractions of 1 and 1.5% (FIGS. 5D and 5E), even thougheach material condition has undergone the same processing method andnanoparticle distribution procedure. Examining the micrographs of FIGS.5A-E in order of increasing loading fraction, from left to right, theappearance of the 0.2 and 0.5 wt % nanocomposites (FIGS. 5B and 5C)appear approximately uniform, indicating homogeneous dispersion of theSiC nanoparticles. Even better uniformity is found in the 0.2 wt %sample, indicating that loadings of less than 0.5 wt % may beparticularly advantageous. With loadings of 1 and 1.5% SiC, however, thenanocomposites exhibit dark and light areas, indicating thatagglomeration occurs in these higher loading nanocomposites. Thisinhomogenaeity may negatively affect the composite properties, asillustrated below.

Dogboned tensile specimens were employed for measurement of the tensileproperties of the two materials using ASTM D638. At least four specimenswere tested for each material condition. FIGS. 6A-D below present thetensile properties of embodiments of the SiC-DGEBPA-AE nanocompositesand corresponding neat matrix.

Examining representative stress-strain curves for the nanocomposite withweight fraction of approximately 0.2% to 1.5% SiC as well as the neatmatrix, FIG. 6A, the un-reinforced neat matrix is observed to fail in abrittle manner, with the stress-strain curve rising linearly to failure.In contrast, nanocomposites containing 0.2-0.4% SiC nanoparticlesexhibit yielding prior to failure, as illustrated by the non-linearportion of the curves at elevated values of stress. Nanocompositescontaining 1% and 1.5% exhibit brittle failure, similar to the neatmatrix.

FIGS. 6B-D compare the ultimate strength, strain to failure, and elasticmodulus of the SiC reinforced nanocomposite compared to the baselineneat matrix, illustrating that the ultimate strength and strain tofailure of the nanocomposites increase significantly with lownanoparticle loadings. In the range of nanoparticle loadings from0.2-0.5%, the ultimate strength rises from about 35 MPa in the neatmatrix to about 57-60 MPa in the range of 0.2-0.5% loadings, an increaseof more than about 70% over the un-reinforced matrix. Thus, generallyfor a loading of about 0.4 wt %, the ultimate strength may increase byabout 70% or more compared to the un-reinforced matrix. In otherembodiments, any suitable nanoparticle loading that causes an increasein ultimate strength of about 10% or more, more preferably about 25% ormore, and even more preferably about 50% or more, may be utilized. Thestrain to failure is also found to benefit over this range ofnanoparticle loadings, increasing to about 0.024 at 0.5% loading fromabout 0.011 in the neat matrix, an increase of more than 200% over theun-reinforced matrix, an increase of over 150%. In other embodiments,any suitable nanoparticle loading that causes an increase in strain tofailure of about 50% or more may be utilized.

Beyond approximately 0.5% loadings, both strength and failure strain arefound to diminish with increased SiC loading, falling below the valuesof the neat matrix at a loading of 1.5%. In fact, the predicted curvesshown in FIGS. 6C and 6D indicate that even at about 0.5% loading,strength and strain to failure may begin to decrease; thus, loadingsless than about 0.5% wt %, e.g., up to about 0.4 wt %, may beadvantageous. For example, at about 1% SiC loading FIGS. 6C and 6D showapproximately no change in strength and strain to failure, and at about1.5% SiC loading there is about a −55% and −50% drop in strength andstrain-to-failure, respectively, compared to the neat matrix. Further,over the range of SiC loadings investigated, the elastic modulus remainsapproximately constant with the increase in nanoparticles, decreasingonly slightly, approximately 14%, when initially added to the polymermatrix. Thus, low loadings of SiC nanoparticles, for example betweenabout 0.2-0.5%, and even between about 0.2 and 0.4%, significantlyimprove the tensile properties of polymer nanocomposites.

An exemplifying SENB fracture specimen 304 is illustrated in FIG. 3B. Inone embodiment, the fracture specimen 304 has a nominal width of about25.4 mm, a span length of about 112 mm, and an average thickness ofapproximately 6.4 mm. A notch 306 of approximately 12 mm in length and1.5 mm in width is cut using a milling cutter and sharpened using arazor blade tapped into the notch. The SENB specimens 304 so providedhave a nominal crack length to specimen width ratio a/W, ofapproximately 0.5, satisfying the ASTM standard's requirement for avalid SENB test. At least four such specimens were tested for eachmaterial condition.

K_(IC), and G_(IC) values for the nanocomposites and neat matrix as afunction of nanoparticle loading are presented in FIGS. 7A-B. Ingeneral, improvements in both K_(IC), and G_(IC) values are observedwith nanoparticle loadings up to 0.2%. K_(IC) rises from approximately0.8 MPa m^(1/2) in the neat matrix to approximately 1.5 MPa m^(1/2) inthe 0.2%-SiC nanocomposite, a nearly two-fold rise. Similarly, G_(IC)increases from approximately 200 J/m² to 750 J/m², over the same range,a nearly four-fold rise. Beyond approximately 0.2% nanoparticleinclusion, both K_(IC) and G_(IC) are found to fall back toapproximately the value of the neat matrix. Fracture tests, therefore,demonstrate that significant improvements in the fracture toughness andcritical strain energy release rate of epoxy can be obtained with lowloadings of nanoparticles, particularly less than approximately 0.2%. Inone embodiment, any suitable nanoparticle loading that causes about atwo fold or more rise in G_(IC), and/or about a 20% or more rise inK_(IC), may be utilized.

These mechanical measurements are further supported by SEM micrographsof the fracture surfaces of the two composites, FIGS. 8A-C. Themicrograph of the neat epoxy surface (FIG. 8A) shows a very shiny orglossy surface suggesting that brittle, fast propagation of fracture hastaken place. The fracture surface of 0.2% SiC reinforced specimen, FIG.8B) in contrast, is duller. Taken with the mechanical measurements,particularly the stress-strain curves demonstrating plastic yielding,this result illustrates that a more ductile or damage tolerant failureprocess has occurred in this nanocomposite than in the neat matrix.Furthermore, the fracture surface possesses a very uniform appearance,indicating a lack of agglomerated nanoparticles which would otherwiseact as failure initiation sites. Examining the fracture surface of thenanocomposite containing 1% SiC nanoparticles, FIG. 8C, the surfaceappears non-uniform, with dark, generally circular irregularities ofagglomerated SiC particles 242. The micrograph further illustratesstriations 802 in the fracture surface, consistent with what would beexpected from brittle fracture. Thus at increased nanoparticleconcentrations, nanoparticle dispersion becomes more difficult toachieve, resulting in agglomerates 242 which act as defects andstriations 802 or stress concentration sites which promote crackpropagation and give rise to brittle fracture of nanocomposites.

These mechanical testing and microscopy results demonstrate that lowloading fractions of SiC nanoparticles provide composites withsubstantially dispersed reinforcement having improved toughness, tensilestrength, and strain to failure.

Bulk Nanocomposites: TiO₂ Nanoparticle Reinforced Nanocomposites

Investigations below illustrate the effects of low loadings of TiO₂nanoparticles on the tensile and fracture properties of nanocompositesand comparable un-reinforced neat matrices. Polyester resin is mixedwith TiO₂ nanoparticles (Accumet Materials Co.). The TiO₂ nanoparticlespossess a density of about 0.04-0.06 g/cm³, a surface area of 190-290m²/g, and a size of approximately 15 nm. TiO₂ loadings of 0.2% areinvestigated in relation to neat polyester matrix only materials. Itwill be appreciated that other TiO₂ loadings may also be utilized, suchas in the range of approximately 0.1% to 0.5%. The TiO₂ nanoparticlesare added to the polyester resin and the mixture stirred using themagnetic mixer approximately 15 hours at 700 rpm in order to dispersethe nanoparticles within the matrix resin. Subsequently, the hardener isadded to the homogeneous mixture, in an amount so as to provide an epoxyto hardener ratio of approximately 30 mL to 10 drops. Following additionof the hardener, the mixture is stirred manually for approximately 5minutes at a linear speed of about 1 cm/sec. Following mixing, themixture is poured into a polished aluminum mold and allowed to cure inair at room temperature for approximately 4 hours. To quantify theeffect of the nanoparticles loading on the performance of the composite,a polyester panel without nanoparticles is also manufactured using thesame procedure.

FIGS. 9A-B illustrate the appearance of an as-processed neat matrix (9A)and a 0.2% TiO₂-reinforced nanocomposite (9B). It is apparent from themicrographs that the nanocomposite presents a white, opaque appearance,while the neat matrix is approximately transparent. In one aspect, thisresult demonstrates that even small loadings of nanoparticles aresufficient to change the optical properties of the nanocomposite fromthose of the neat matrix. In another aspect, this result indicates thatthe mixing procedure described above provides substantially homogeneousdispersion of the nanoparticles within the resin, as the appearance ofthe nanocomposite is approximately uniform.

Dogboned tensile specimens were employed for measurement of the tensileproperties of the two materials using ASTM D638. At least four specimenswere tested for each material condition. FIGS. 10A-C below present thetensile properties of embodiments of the TiO₂-Polyester nanocompositesand corresponding neat matrix.

Examining representative stress-strain curves for the nanocomposite, aswell as the neat matrix, FIG. 10A, the un-reinforced neat matrix isobserved to fail in a brittle manner, with the stress-strain curverising linearly to failure. In contrast, nanocomposites containing 0.2%TiO₂ nanoparticles exhibit yielding prior to failure, as illustrated bythe non-linear portion of the curves at elevated values of stress.

FIGS. 10B-C presents the ultimate strength and strain to failure of theTiO₂-polyester nanocomposites containing 0.2% TiO₂ compared to thebaseline neat polyester matrix as well as the relative percentage changein the nanocomposite properties with respect to the baseline. Ingeneral, the ultimate strength and strain to failure of thenanocomposites are substantially higher than that obtained in the neatmatrix. Specifically, the ultimate strength of the nanocomposite isapproximately 25% higher than the neat matrix, approximately 47.43±3.2MPa, compared to approximately 38±0.05 MPa. Furthermore, the failurestrain is almost doubled, from about 0.014±0.001 in the neat matrix toapproximately 0.028±0.004 in the 0.02-TiO₂ nanocomposite. In otherembodiments, any suitable nanoparticle loading that causes an increasein ultimate strength of about 10% or more, or an increase in strain tofailure of about 50% or more, may be utilized. Addition of nanoparticlesto the neat matrix is found to not significantly affect the Young'smodulus of the nanocomposite (about 10% reduction), with the modulus ofthe neat matrix determined to be approximately 3.15±0.02 GPa to the2.83±0.4 GPa measured in the nanocomposite.

SENB fracture specimens 304 of TiO₂ reinforced nanocomposites areprovided as schematically illustrated in FIG. 3B. In one embodiment, thefracture specimen 304 has a nominal width of about 12.5 mm, a spanlength of about 56 mm, and an average thickness of approximately 3.2 mm.A notch 306 of approximately 6 mm in length and 1.5 mm in width is cutusing a milling cutter and sharpened using a razor blade tapped into thenotch. The SENB specimens 304 so provided have a nominal crack length tospecimen width ratio a/W, of approximately 0.5, satisfying the ASTMstandard's requirement for a valid SENB test. At least four suchspecimens were tested for each material condition

FIGS. 11A-B illustrate the results of SENB fracture tests of anembodiment of a 0.2% TiO₂-polyester nanocomposites and comparable neatpolyester matrix. With introduction of the 0.2% TiO₂, K_(IC) of thenanocomposite increases from about 0.7 MPa m^(1/2) to about 1.17 MPam^(1/2), a roughly 65% increase and more than about a 50% increase overthe neat matrix. Further, G_(IC) of the nanocomposite increases morethan three fold, from approximately 129 J/m² for the neat matrix toapproximately 546 J/m² for the nanocomposite. In other embodiments, anysuitable nanoparticle loading that causes about a two fold or more risein G_(IC), and/or about a 20% or more rise in K_(IC), may be utilized.

Bulk Nanocomposites: CNT Reinforced Nanocomposites

Investigations below illustrate the effect of inclusion of low loadingsof SWNTs on the tensile and fracture properties of nanocomposites andcomparable un-reinforced neat polymers. An epoxy based on DGEPBA-AE ismixed with single walled CNTs (SWNTs) (CarboLex Corporation). Thesecarbon nanotubes have an average diameter of approximately 1.4 nm andare found in “ropes” which are typically about 20 nm in diameter orapproximately 50 tubes per rope with lengths of 2-5 microns. Two weightfractions of CNTs are investigated: 0.02, and 0.1 wt %. Owing to thehigh degree of adhesion between nanotubes, SWNTs are difficult toseparate into single units from aggregates and clusters within epoxy, asepoxy possesses a high viscosity. To circumvent this difficulty, SWNTsare first dispersed within ethyl alcohol (EtOH). The nanotubes are addedto EtOH and subjected to a sonication treatment for approximately 5hours at room temperature. The dispersed EtOH-SWNT solution is mixed atroom temperature with the resin using a magnetic hot plate stirrer atapproximately 700 rpm for about 15 hours. Further stirring atapproximately the same speed is continued, in addition to heating at aconstant temperature of about 80° C., for about 45 minutes in order toevaporate the highly volatile EtOH from resin. The mixture issubsequently subjected to a second sonication treatment forapproximately 25 min. The hardener is then added to the nanotube-epoxymixture in an approximately 3:1 ratio of epoxy-to-hardener.Subsequently, the nanotube-epoxy-hardener mixture is subjected to slowmanual stirring for about 20-30 minutes at a linear speed ofapproximately 1 cm/sec. This mixing process provides a substantiallyhomogeneously dispersed mixture of SWNT and resin. Following mixing, themixture is poured into a polished aluminum mold and allowed to cure inair at room temperature for approximately 5 hours. Subsequently, thecured composites are post-cured at approximately 120° C. for 1 hour inair. To quantify the effect of nanotube loading on the performance ofthe composite, an epoxy panel without nanotubes is also manufacturedusing the same procedure.

FIGS. 12A-B illustrate the appearance of the as-processed neat matrixand an approximately 0.02% nanotube-reinforced nanocomposite. It isapparent from the micrographs of FIG. 12B, the nanocomposite is coloredin appearance, while the neat matrix is approximately transparent, FIG.12A. In one aspect, this result demonstrates that even small loadings ofnanoparticles are sufficient to change the optical properties of thenanocomposite from those of the neat matrix to translucent or opaque. Inanother aspect, this result indicates that the mixing proceduredescribed above provides substantially homogeneous dispersion of thenanoparticles within the resin, as the appearance of the nanocompositeis approximately uniform.

Dogboned tensile specimens were employed for measurement of the tensileproperties of the two materials using ASTM D638. At least four specimenswere tested for each material condition. FIGS. 13A-C below present thetensile properties of the CNT-DGEBPA-AE nanocomposites and correspondingneat matrix.

Examining representative stress-strain curves for the nanocomposite, aswell as the neat matrix, FIG. 13A, the un-reinforced neat matrix isobserved to fail in a brittle manner, with the stress-strain curverising linearly to failure. In contrast, nanocomposites containing 0.2%CNTs nanoparticles exhibit yielding prior to failure, as illustrated bythe non-linear portion of the curves at elevated values of stress

FIGS. 13B-C presents the ultimate strength and strain to failure ofembodiments of the CNT-DGEBPA-AE nanocomposites containing about0.02%-0.1% CNTs compared to the baseline neat matrix as well as therelative percentage change in the nanocomposite properties with respectto the baseline.

As illustrated in FIGS. 13B-C, the neat matrix exhibits a brittleelastic response, with the stress-strain response rising approximatelylinearly to failure. In contrast, the nanotube-reinforced nanocompositesillustrate yielding prior to failure. Furthermore, the ultimate strengthand strain to failure of the nanocomposites are higher than thatobtained in the neat matrix: ultimate strength of the nanocomposite isapproximately 15% higher than the neat matrix, approximately 40.72±4.5MPa, compared to 35.9±1.05 MPa. The failure strain also illustratesimprovement, rising approximately 150% higher to 2.77%, compared to1.09%. In other embodiments, any suitable nanotube loading that causesan increase in ultimate strength of about 10% or more, and/or anincrease in strain to failure of about 50% or more, may be utilized. TheYoung's modulus of the nanocomposite is found to be reducedapproximately 45%, with the modulus of the neat matrix measured to beapproximately 3.5 GPa to the 2.04 GPa measured in the nanocomposite.

SENB fracture specimens 304 of CNT reinforced nanocomposites areprovided as schematically illustrated in FIG. 3B. The SENB specimens 304tested possess a width of approximately 12.5 mm, a span length ofapproximately 56 mm, and an average thickness of approximately 3.2 mm. Anotch 306 of approximately 6 mm in length and 1.5 mm width is cut intothe specimens using a milling cutter and sharpened using a razor bladetapped into the notch. So configured, the crack length to width ratio,a/W, is approximately 0.5, satisfying the ASTM requirements for a validSENB fracture test.

FIG. 14A-B illustrates the results of SENB fracture tests of embodimentsof nanotube-epoxy nanocomposites and the neat matrix. With introductionof the 0.02-0.1% nanotubes, K_(IC) of the nanocomposite more thandoubled, exhibiting a roughly three fold increase, from about 0.7 MPam^(1/2) in the neat matrix, to about 2.2 MPa m^(1/2) in thenanocomposite. Further, G_(IC) of the nanocomposite increases by morethan a factor of 8 and almost ten fold over the neat matrix, fromapproximately 200 J/m² for the neat matrix to approximately 1900 J/m²for the nanocomposite.

EXAMPLES Nanocomposite Coatings

In further examples, testing is also performed on polymer coatingsreinforced using nanoparticles and their corresponding un-reinforcedcounterparts in order to illustrate the property improvements which maybe achieved in these coatings over un-reinforced coatings. In theexamples below, polymer coatings comprising nail polish areinvestigated. In particular, chemical resistance and mechanicalresistance to wear and chipping are examined. While the examples belowillustrate nail polishes reinforced with TiO₂ nanoparticles, it may beunderstood that other nanoparticles which include, but are not limitedto SiC, ZnO, Y₂O₃, and C, as well as CNTs may be utilized in alternativeembodiments of the invention. In preferred embodiments of the invention,a single nanoparticles or nanotubes species is provided as areinforcement.

In general, when applying nail polish, three coatings are employed, abase coating which is applied directly upon the nail, a main colorcoating which is applied upon the base coating, and a top coating whichis applied upon the main color. To test the influence of nanoparticlereinforcements on nail polish coatings, representative, commerciallyavailable nail polish products are employed. For example, base and topcoatings comprising REVLON® FIRMA NAIL and main color coatingscomprising REVLON® COLOR ILLUSION, having a metallic red color, areexamined. REVLON® FIRMA NAIL comprises combinations of the followingcompounds: Isopropanol, Acetyl tributyl citrate, Citric acid, Butylacetate, Sucrose acetate isobutyrate, Ethyl acetate, Heptane,Etocrylene, Malic acid, Bismuth oxychloride, Nitrocellulose, Sucrosebenzoate, 2-Propenoic acid, polymer withN-(1,1-dimethylethyl)-2-propenamide and ethyl 2-propenoate,Stearalkonium hectorite, and Stearalkonium bentonite. REVLON® COLORILLUSION, is a nail enamel, generally comprising film forming agents(e.g. Nitrocellulose), resins and plasticizers, solvents, and coloringagents. Further details regarding the formulation of nail polishutilized in embodiments of the invention discussed above may be found inU.S. Patent Application Publication 2005/0220730 to Malnou, et al., theentirety of which is hereby incorporated by reference.

Sample Preparation: Nanocomposite Coatings

TiO₂ nanoparticles are added to each nail polish coating separately anddispersed using the magnetic stirring technique discussed above. TheTiO₂ nanoparticles possess a particle size of approximately 15 nm, adensity in the range of about 0.04-0.06 g/cm³, and a surface area ofapproximately 190-290 m²/g. Preferably, the nanoparticles may beprovided in a quantity as described above, for example about 0.1 wt % to0.4 wt %, more preferably about 0.2 wt %. To distribute thenanoparticles uniformly within the coatings, an octagonal stirring barhaving a length of approximately ½ inch and a diameter of approximately⅛ inch is used for magnetic stirring. Stirring is performed in a closedcontainer at a speed in the range of about 220-280 r.p.m. forapproximately 15 hours.

Examining the as-mixed coatings, several observations were noted. In oneaspect, after mixing, it is observed that the main color issubstantially unchanged by the nanoparticle reinforcement. In anotheraspect, the color of the base and top coatings exhibited modestwhitening. It is further noted, however, that the change in color of thebase and top coatings did not substantially alter the appearance of thefinal nail polish color when the three coatings were layered upon oneanother. While nanoparticle reinforcements of nail polishes havepreviously been performed, such as in U.S. patent application No.2005/0220730, the influence of the reinforcement on the color of theresulting polish has not previously been appreciated. In embodiments ofthe present invention, the loading fraction of the nanomaterialreinforcement is selected so as to not substantially influence the colorof the resulting polish. It is additionally noted that the nanoparticlereinforcement had substantially no influence on the consistency of thenail polish or ease of application.

The prepared coatings are applied to artificial finger nails fortesting. A commercially available artificial nail, Fing'rs® is employed.These artificial nails possess a length of approximately 19 mm and acurvature of about 6.5 mm. The coatings are applied to the nails usingappropriate nail polish brushes in two different configurations. In thefirst configuration, a layer of either the base coating/top coating orthe main coating is applied to the nail. Four sets of nails are coatedin this manner for testing, with two sets using nail polish reinforcedwith nanoparticles and two sets using unreinforced nail polish. Each setpossessed three nails.

In the second configuration, one layer of base coating, two layers ofmain color coating, and one layer of top coating are applied, while thelayers which possess the nanoparticle reinforcement are varied. A dryingtime of approximately 10 minutes is employed after each coating layer isapplied. In a first sample set, each coating is without nanoparticlereinforcement. In a second sample set, only the base layer possesses thenanoparticle reinforcement. In a third sample set, the base coating andthe two main color coating layers possess the nanoparticlereinforcement. In a fourth sample set, the nanoparticle reinforcement ispresent within all the coating layers. Nails prepared in this manner areobserved to demonstrate substantially no difference in appearance,despite the variation in nanoparticle reinforcement.

In preparation for testing, the coated nails are glued to aluminum barshaving diameters and curvatures approximately equal to that of thenails. Approximately ⅔ of the length of each nail, about 12.5 mm, isaffixed to the aluminum bars, leaving about ⅓ of the nail length, orapproximately 6.5 mm, extending past the end of the bars.

Chemical and Mechanical Testing: Nanocomposite Coatings

To examine the performance of nanoparticle reinforced nail polish,chemical and mechanical tests are performed to simulate environments andactivities that human nails are commonly exposed to and engaged in.Chemical tests are performed by soaking the prepared nails in commonhousehold chemicals. A concentration of approximately 50 wt % is usedfor all chemicals in order to provide a relatively conservativesimulation of chemical exposure. Mechanical testing is performed bysimulating a typing action. The aluminum bars are used as fingers tocyclically press the nail tips into and out of contact with a computerkeyboard. The testing times are illustrated in Table II below.

TABLE II Testing Matrix Concentration Daily Exposure Life Cycle ExposureTime/Cycle Test Brand (wt %) Time (min) Time (h) (min) Chemical DishWashing Dawn ® 50 48 22.5 150 Liquid Liquid Hand Soap Ivory ® 16 7.5 50Hair Shampoo Suave ® 13 6 40 Hair Conditioner Suave ® 6.5 3 20 MakeupRemover Neutrogena 1.5 0.75 5 Deepclean ® TOTAL TOTAL TOTAL 85 39.75 265(4.42 h) Mechanical TOTAL TOTAL Typing 19 9 60 (1 h)  Chemical and mechanical tests to evaluate the performance ofnanoparticle reinforced nail polish are conducted on the polished nailsbased upon an estimation of a daily exposure time for each chemical ormechanical activity (typing). Respectively, the daily exposure times aretaken to be approximately 48, 16, 13, 6.5, and 1.5 minutes for dishwashing liquid, liquid hand soap, hair shampoo, hair conditioner, andmakeup remover. Similarly, the daily time spent typing is taken to beapproximately 19 minutes.

The daily exposure time is used to estimate the total exposure time overthe life cycle of the nail polish. Taking the life cycle of the nails tobe approximately four weeks, the daily exposure time is multiplied by 28days to yield approximate values for the total exposure time over thelife cycle of the nails. These values are taken to be, respectively,22.5, 7.5, 6, 3, and 0.75 hours for dish washing liquid, liquid handsoap, hair shampoo, hair conditioner, and makeup remover.

Accelerated testing is performed to simulate these total exposure times.The total exposure time for each chemical and mechanical activity isdivided into 9 testing cycles. In a cycle, the nails are successivelysoaked in dish washing liquid, liquid hand soap, hair shampoo, hairconditioner, and makeup remover, for approximately 150, 50, 40, 20, and5 minutes, respectively. Between each exposure, the nails are allowed todry for approximately 2 hours. Nails are wiped and dried after eachchemical exposure using paper towels for the last five testing cycles.Following each chemical exposure cycle, the nails are used to typecontinuously for approximately one hour, cyclically bringing the nailsin and out of contact with a keyboard in order to simulate a typingaction on a computer keyboard. Upon completion of all the testingcycles, the appearance of the nails is examined.

Examination and comparison of the nails having single layernanocomposite coatings, multiple layer nanocomposite coatings, and theirunreinforced counterparts revealed that nail polishes with nanoparticlereinforcements often exhibit superior performance to unreinforced nailpolishes. FIG. 15A-B illustrates artificial nails 1500 having a singlelayer of main color coating which are unreinforced (FIG. 15A) andreinforced with the TiO₂ nanoparticles (FIG. 15B). It is observed thatthe unreinforced nail polish exhibits scratches 1502 on the surface ofthe nail after testing, as well as chipping 1504 at the tip of the nail.In contrast, nails painted with nanoparticle reinforced nail polishappear substantially free of such scratching or chipping. These results,therefore, demonstrate that the nanoparticle reinforcement enhances thedurability, and thus the useful lifetime, of single nail polishcoatings.

And while the single layer main color coatings tested using thetechniques, setup, and durations discussed above showed cleardifferences in terms of scratches and chipping with and withoutnanoparticles, the multiple layer nail polish coatings with and withoutnanoparticles did not illustrate such differences. Further testing maybe required to clearly identify improvements.

These findings demonstrate the substantial gains which can be achievedin strength and strain to failure, as well as in toughening upon lowpercentage reinforcement of bulk polymer matrices with homogenouslydistributed nanoparticles and nanotubes. The results further show thatnail polish applications may benefit from the use of low loadingfractions of reinforcing nanoparticles. It should be noted that U.SPatent Application Publication 2005/0220730 fails to appreciate theadvantages and features which may be obtained in nail polishes and otherpolymer systems reinforced with nanomaterials in the quantities and toobtain the properties as discussed above, and for example in the loadingrange of approximately 0.01 to 0.4 wt %. In particular, the benefits tothe strength and toughness of nanocomposites so reinforced have notpreviously been recognized.

Thus, certain embodiments of the present invention provide systems andmethods of fabricating nanocomposites having low weight fractions ofnanoparticles and nanotubes which are substantially uniformlydistributed. Advantageously, the disclosed mixing technique, including amagnetic mixing step, provide a substantially uniform distribution ofthe nanotubes and nanoparticles which can provide significantly enhancedtensile strength, strain to failure, and fracture toughness over neatmatrices alone.

Although the foregoing description has shown, described, and pointed outthe fundamental novel features of the present teachings, it will beunderstood that various omissions, substitutions, and changes in theform of the detail of the apparatus as illustrated, as well as the usesthereof, may be made by those skilled in the art, without departing fromthe scope of the present teachings. Consequently, the scope of thepresent teachings should not be limited to the foregoing discussion, butshould be defined by the appended claims.

1. A composite material, comprising: a matrix material selected from thegroup consisting of epoxies and polyesters; and a reinforcement materialcomprising at least one of nanoparticles selected from the groupconsisting of TiO₂, ZnO, SiC, Y₂O₃, C and carbon nanotubes having atleast one dimension of about 100 nm or less dispersed within the matrixmaterial, wherein the reinforcement is present in a concentrationbetween approximately 0.01% and 0.4% on the basis of the weight of thecomposite material so as to improve at least one of strength andtoughness of the composite.
 2. A nanomaterial-reinforced nail polishcomprising the composite material of claim 1, wherein the reinforcementmaterial is present in a concentration between approximately 0.2 to 0.4wt %.
 3. The composite material of claim 1, wherein the reinforcementmaterial is present in a concentration between approximately 0.1% to0.4% on the basis of weight of the composite material.
 4. The compositematerial of claim 1, wherein the matrix material comprises an epoxy andthe reinforcement material comprises SiC.
 5. The composite material ofclaim 4, wherein the SiC reinforcement material is provided in aconcentration between about 0.2% and 0.4% on the basis of the weight ofthe composite material.
 6. The composite material of claim 1, whereinthe matrix material comprises a polyester resin and the reinforcementmaterial comprises TiO₂.
 7. The composite material of claim 6, whereinthe TiO₂ reinforcement material is provided in a concentration of about0.2% on the basis of the weight of the composite material.
 8. Thecomposite material of claim 1, wherein the reinforcement materialcomprises carbon nanotubes in a concentration of about 0.01% to 0.1% onthe basis of the weight of the composite material.
 9. The compositematerial of claim 1, wherein the matrix comprises a high temperatureepoxy based on diglycidyl ether of bisphenol A and alkylglycidyl ether,a ceramic based on a pre-ceramic polymer, or polyester.
 10. Thecomposite material of claim 1, wherein the reinforcement causes anincrease in ultimate strength of the composite of about 10% or morecompared to an unreinforced matrix.
 11. The composite material of claim1, wherein the reinforcement causes an increase in strain to failure ofthe composite of about 50% or more compared to an unreinforced matrix.12. The composite material of claim 1, wherein the reinforcement causesan increase in G_(IC) of about two-fold or more compared to anunreinforced matrix.
 13. The composite material of claim 1, wherein thereinforcement causes an increase in K_(IC) of about 20% or more comparedto an unreinforced matrix.
 14. The composite material of claim 1,wherein the matrix material comprises a nail polish and thereinforcement material comprises TiO₂.
 15. The composite material ofclaim 14, wherein the TiO₂ reinforcement material is provided in aconcentration of about 0.2% on the basis of the weight of the compositematerial.
 16. The nail polish of claim 2, wherein color of the nailpolish is substantially unchanged by the presence of the reinforcementmaterial.